Personal computer systems in general and IBM personal computers in particular have attained widespread use for providing computer power to many segments of today's modern society. Personal computer systems can usually be defined as a desk top, floor standing, or portable microcomputer that consists of a system unit having a single system processor and associated volatile and non-volatile memory, a display monitor, a keyboard, one or more diskette drives, a fixed disk storage, and an optional printer. One of the distinguishing characteristics of these systems is the use of a motherboard or system planar to electrically connect these components together. These systems are designed primarily to give independent computing power to a single user and are inexpensively priced for purchase by individuals or small businesses. Examples of such personal computer systems are IBM's PERSONAL COMPUTER AT and IBM's PERSONAL SYSTEM/2 Models 25, 30, 50, 60, 70 and 80.
These systems can be classified into two general families. The first family, usually referred to as Family I Models, use a bus architecture exemplified by the IBM PERSONAL COMPUTER AT and other "IBM compatible" machines. The second family, referred to as Family II Models, use IBM's MICRO CHANNEL bus architecture exemplified by IBM's PERSONAL SYSTEM/2 Models 50 through 80. The Family I models typically have used the popular Intel Corporation 8088 or 8086 microprocessor as the system processor. These processors have the ability to address one megabyte of memory. The Family II models typically use the high speed Intel Corporation 80286, 80386, and 80486 microprocessors which can operate in a real mode to emulate the slower speed Intel Corporation 8086 microprocessor or a protected mode which extends the addressing range from 1 megabyte to 4 Gigabytes for some models. In essence, the real mode feature of the 80286, 80386, and 80486 processors provide hardware compatibility with software written for the 8086 and 8088 microprocessors.
In such personal computers, it has been conventional to follow good engineering design practice for the circuit board which typically mounts the system microprocessor and provides the conductive pathways by which digital signals flow to and from the microprocessor. Such circuit boards are known as system boards, motherboards or planars. A typical motherboard is manufactured as a multiple layer board having a layout of circuit components and ground areas designed to place input/output signal handling circuits adjacent the entry point of such signals into the pathways provided in the planar, provide appropriate components for suppression of the effects of electromagnetic interference, and establish a "clean" or "quiet" ground grid adjacent such an entry point. Persons skilled in the arts of planar design have been taught to follow such design practices by those who have prepared engineering papers and monographs on such subjects, and it is deemed not necessary to provide a detailed teaching of such skills here.
While such design approaches have enabled the manufacture and use of personal computers appropriately shielded against the possibly adverse effects of electromagnetic interference (EMI), thereby providing electromagnetic compatibility (EMC), a planar so designed is restricted to the particular input/output pathways for which the board was originally designed. Variation, and enhancement after the original design has been "frozen", is extremely difficult if not impossible without major modification of the planar. Thus a planar design, once established, is not readily susceptible to improvement and addition of new input/output capabilities or characteristics.